Incandescent light source



Nov. 4, 1958 F. E. WILLIAMS ET AL 2,859,369

INCANDE S CENT LIGHT SOURCE 2 Sheets-Sheet 1 Filed June 15, 1954 Inventors: Ferd. E. Williams, eter D. Johnson,

by w) Then" Attorn e y.

Nov. 4, 1958 Filed June 15, 1954 F. E. WILLIAMS ET AL INCANDESCENT LIGHT SOURCE 2 Sheets-Sheet 2 Inventors Fer-d E.Williams eter D. Johnson,

by 4 iuwe/ Their Attorney.

2,859,369 Patented Nov. 4, 1 958- United States Patent ()fifice INCANDESCENT LIGHT SOURCE Ferd E. Williams and Peter D. Johnson, Schenectady,

N. Y., assignors to General Electric Company, a corporation of New York Application JunelS, .1954, Serial No. 436,836

7 Claims. (CL 313-113 'This-invention relates to incandescent light sources as, for instance, incandescent lamps. 'More particularly, the invention :relates to incandescent lamps which are characterized by increased light production with reduced power input.

It is well known that, of the energy radiated from a :modern tungsten filament incandescent lamp, only about 10% is in the visible region of the spectrum, the remaining 90% being emitted as heat, primarily in the infra red region. The problem of increasing the visible light output of incandescent lamps for a given power input or for decreasing power input for a given intensity of emitted light has long stood as a challenge in the art.

One object of'this invention is to provide an incandescent lamp which is characterized by increased visible light output at reduced power input.

Another object of the invention is to utilize the infra red radiation from a conventional tungsten filament incandescent lamp to reduce the power input to the filament necessaryfor a given level of visible light intensity.

In accordance with one feature of the invention, there is provided an incandescent light bulb in which a small filament is provided at the geometrical center of one or more spherical surfaces which are coated with a reflecting film and,interior of'the reflecting 'film, with a thin layer which constitutes a Rayleigh scatterer. The reflecting layer functions to reflect infra red .light back to the filament,.thus conserving the heat energy that is dissipated asinfra red light in conventional incandescent lamps. The Rayleigh scattering layer functions to selectively scatter the visible component of radiated light out .of the lamp so that it may be utilized for illumination.

The novel features characteristicof the invention are set forth in the appended claims. The invention itself, however, together with furthenobjects and advantages thereof, may best be understood by referring to the following description taken in connection with the accompany drawings, in which:

Fig. 1 illustrates an incandescentlarnp illustrating one feature of this invention;

Fig. 2 he sectional .view of thelamp of Fig. 1 taken along section I line f22;

Fig. 3 is an enlarged sectional view of a portion of the Wall of'the'larnp' illustrated in Fig. 2; and

'Figs. 4, 5 and 6.are diagrammatic sketches, including ray diagrams, illustrative of various configurations which may be utilized .in alternative embodiments of the invention.

When'lig'ht waves are incident upon a small particle, thelarg'est dimension of which is small in comparison to. .the wavelength of the incidentlight, it is found that the small particle actsas a center of propagation of a new wavetrain, the'direction of propagation of which is dependent uponthe orientation of the particular scattering particle. When waves of light are incident upon a medium comprising a large number of particles, the dimensions of which are small with respect to the wave length of the incident light, a number of wave trains are generated from the individual particles and the incident light is scattered in all directions. This light is said to be scattered rather than reflected, and the laws of reflection are not applicable to the phenomena. It has been found that when light comprising a number of components of varying wave lengths is incident upon a scattering medium, the scattering of the various l ight components varies greatly with wave length. Analytically, the dependence is such that the scattering of the various light components varies inversely in proportion to the fourth power of the wave lengths of the light scattered. This scattering by small particles is known as Rayleigh scattering, and a medium which causes such scattering in a beam of incident light 'may be referred to as a Rayleigh scatterer. Accordingly, a Rayleigh scatterer may be defined as a medium comprising particles the dimensions of which are small with respect to the Wave length of the incident light. This type scatterer is further characterized in that the intensity of scattered light is inversely proportional to the fourth power of the wave length of the light scattered. .Ithas further been found that Rayleigh scattering may be caused by a particulate medium having particle sizes which are larger than the normal size of Rayleighscattering particles if the individual particles are irregular.

in shape and possess a number of surfaces which are very srnall with respect to the wave length of incident light. center of propagation of a scattered wave train. Thus, while it is necessary that, to scatter visible light, each Rayleigh scattering surface be of the order of 500 A.U. in dimension, the particles including the surfaces may be as large as approximately 3500 A. U. ,in dimension.

The phenomena of Rayleigh scattering lends itself admirably well to the problem of increasing the ell}.

ciency of the modern tungsten filament incandescent lamp. The spectral distribution ofthe emission .of ,a

tungsten filament lamp at normal operating temperatures is very'broad; however, this spectral distribution is concentrated primarily in the long wave length or infra red Only approximately 10% of this emission ,is within the visible spectrum. The peakof the emission spectrum of a tungsten filament is located .at approrri region.

mately l-LOQOangstrom units. On .the other hand, "the wave length of the peak of visual sensitivityis approgri- I Applying the Rayleighscattering formula and the inverse fourth power relamately 5500 angstrom units.

tionship, it may be seen that when a beam of light emitted from a tungstenfilament falls upon aRal/leigh scatterer, the peak infra red light will be scattered only $3 as much as the light at the wave length of maximum visual, sensitivity.

In 'Figs. land 2,.there is shown an incandescent lampconstructed in accordance with this invention. ,In ,Fig. 1

the lamp may comprisev an evacuable envelope 1, a base 12,. and a centrally located filament 3 supportedupon-rigid filament'leads 4 and 5. Fig. 2 shows in vertical crosssection the lamp of Fig. 1. The envelope ,1,of ,the.1amp is coated with a composite coatingcomprising reflecting film 6 and Rayleigh scattering layer 7. V V

This construction-may be seen with greater clarityby referring to Fig. 3, where aportion of the surfaceof bulb? envelope 1 'is shown in cross-section. In Fig. 3 the wall of the bulb envelope 1 maybe composed ,of any transparent, material which may be conveniently foim d Thismaterial may con in manufacturing processes. veniently be glass. Upon glass envelope 1 there is do In this case each small surface portion acts as a' 3 posited,.by well-known vapor deposition or evaporation techniques, a very think reflecting film 6, which may conveniently be metallic aluminum, of a thickness of approximately 1 to microns. Interior of reflecting film 6, Rayleigh scattering layer 7 is located. Rayleigh scattering layer 7 may. be composed of any transparent particulate matter, the particle size of which is approximately /2 the wave length of visible light or less and which has an irregular surface including discrete surface portions with dimensions of the order of 500 A. U. or less. This material is not critical as to its chemical composition, so long as the necessary particle size and scattering surface dimensions are satisfied. Rayleigh scattering layer 7 may be deposited upon the interior of reflecting layer 6 by smoking, sputtering, exploding powders, by the decomposition of volatile vapors, or any other method which will deposit a thin layer ofparticulate matter having the necessary particle size and'scattering surface dimension. Some materials which have been found to be suitable for this purpose are magnesium oxide (which may be deposited by smoking with a burning magnesium ribbon), zinc oxide, magnesium carbonate, finely divided silica (obtained by reaction of a silicon tetrahalide with moisture or by burning an organosilicon compound or silane), and finely divided titanium dioxide (obtained by reaction of titanium tetrahalides'with moisture at the proper pressure). In

generalfany material which in bulk form is transparent to visible and infra red light and which can be deposited in a thin layer of particles not over 3,500 angstrom units in dimension having discretesurface portions of approximately 500 U. or'less in dimension is suitable.

Referring now to Fig. 4, it may be seen how, according to this invention, the efliciency of an incandescent lightbulb may be greatly increased. In Fig. 4, light comprisingthe emission spectrum of a tungsten filament, peaked in the vicinity of 11,000 angstrom units in wave length, is emitted from tungsten filament 3 located at the geometrical center of the spherical surface of bulb envelope 1. The interior of bulb envelope 1 has applied thereto a reflecting coating 6 which may conveniently comprise a thin evaporated film of metallic aluminum, and a Rayleigh scattering layer 7 which may conveniently comprise a film of magnesium oxide, produced by the burning of a magnesium ribbon within envelope 1. The thickness of layer 7 may vary from approximately 0.25 to 1.5 micron, or the thickness of from 1 to 4 magnesium' .oxide particles. Rays of infrared light A (shown as dotted lines) emitted by filament 3 pass through Rayleigh scattering layer 6 with negligible scatteri'n'glo'sses. The scattering of infra red rays by Rayleigh scattering layer 7 removes only approximately 5% of the energy contained therein from the beam. The infra red rays on passing through Rayleigh scattering layer 7 are reflected from reflecting layer 6 back to the centrally located filament 3 and raise the filament temperature, or lower the necessary filament input power to maintain the filament at a constant temperature. This infra red light contains approximately 90% of the energy radiated from filament 3. In order that infra red light be reflected back to the filament by reflecting layer 7 itis necessary that the reflecting surface 1 include one ormore spherical surfaces, the center of curvature of which is located at the filament 3. It is not necessary that reflecting film 6 and Rayleigh scattering layer 7 be applied to the interior surface of bulb envelope 1. These layers' need only' be applied to one or more spherical surfaces having centers of curvature located at the filament, and may be applied to such'surfaces whichmay be independent, of the bulb envelope. In this illustrative embodiment, the bulb envelope 1 is shown to be a single continuous spherical surface with layers 6 and 7 deposited thereon for simplicity. In a conventional incandescent light bulb the total infra red radiation passes through the bulb wall and is lost, thus limiting the efliciency of the lamp to approximately 10%. With the arrangement of this invention, approximately of the infra red energy is returned to the filament and the efiiciency of the lamp is limited only by the absorptivity of the filament.

Rays of visible light of wave lengths from 4,000 to 7,000 angstrom units, represented as solid lines B in Fig. 4, are radiated from filament 3 and impinge upon Rayleigh scattering layer 7. Since the wave length of the maximum sensitivity of visible light is approximately 5,500 angstrom units, or one half as great as the peak infra red rays radiated from the filament, the visible light will be scattered by Rayleigh scattering layer 7 a factor of approximately 16 times greater than infra red rays. It has been determined that coatings of magnesium oxide deposited according to this invention, by the burning of x a magnesium ribbon within bulb envelope 1, scatter approximately 80% of the visible light for a magnesium oxide layer thickness of approximately 3000 angstrom units. A Rayleigh scattering film of magnesium oxide approximately 6000 angstrom units in thickness scatters approximately 92% of the visible light. Thicker layers of Rayleigh scattering material would approach complete scattering of visible light; however, as the thickness of a the scattering layer increases, the scattering layer tends to scatter increasing amounts of infra red energyalso, and therefore decreases the efliciency of the lamp.

It has been found'that the Rayleigh scattering layers having thicknesses of up to 4000 angstromv units neither scatter nor absorb any appreciable amount of infra red radiation. Additionally, it has been determined that sufficient scattering of visible light may be obtained. from a Rayleigh scattering layer comprising magnesium oxide particles for any thickness in excess of 1000 angstrom units. Therefore, it maybe seen that, if the Rayleigh scattering layer 7 comprises a layer of magnesium oxide particles varying from 1000 to 4000 angstromunits in thickness, this layer will scatter and, absorb a negligible amount 'of infra red radiation from the filament but will, nevertheless, scatter a very high percentage of in-[ cident Visible light emitted from the filament. When rays B of visible light from filament 3 are scattered by Rayleigh scattering layer 7,' theserays will scatter diffusely throughout the volume of the incandescent bulb and may be scattered by an infinite numberof scattering processes until the rays are scattered at a proper angle to pass through the clear portion 8 of bulb envelope I which is uncoated and transparent. Due to the fact that Rayleigh scattering is a non-dissipative, non-energy absorbing process, when ray B finally emerges through;

the clear portion 8 of "bulb envelope 1, the intensity of the beam will be the same as the original intensity before scattering. Thus, it may be seen that the invention contemplates the two-fold improvement in incandescent lamps of conserving infra red radiation from an'incandescent filament to contribute to the heating of the filament, and the conservation of visible light radiated by a filament. i

In Fig. 5 there is shown an alternativ e embodiment of V the invention, wherein the aperture in the reflecting and Rayleigh scattering layers has been expanded so that only /2 of the surface of the sphere of bulb envelope 1' is coated with these layers. Since visible and infra red light may escape unhampered through the'uncoated portion 8 of bulb envelope 1 of the embodiment of Fig. 5, e

the maximum efficiency of this embodiment is approximately only half as great as the embodiment of Fig. 4.

Nevertheless, tests have proven that a bulb constructed according to this embodiment of the invention is as efl'icient as a conventional tungsten filament incandescent lamp of the same wattage and geometrical configuration.

In Fig. 6 there is shown another alternative embodir ment of the invention. The incandescent lamp bulb eavelope 1 of Fig. 6 comprises two spherical segmentsof difierent radii, both having their center of curvatures located at the filament 3. The interior surfaces of these spherical surfaces are coated with a reflecting film 6 and a Rayleigh scattering layer 7 as in the other embodiments. Rays of infra red energy pass through Rayleigh scatterer 7, losing negligible amounts of energy through scattering, and are reflected back to filament 3 by reflecting layer 6, thus conserving and utilizing infra red energy. Rays of visible light B, shown in solid lines, are diifusely scattered by Rayleigh scattering layer 7, and may be scattered one or more times until they are scattered out through uncoated portions 8' of bulb envelope 1. This configuration presents the maximum possible efficiency for an incandescent bulb constructed according to this invention. In this. embodiment the entire surface of bulb envelope 1, as seen by filament 3, is covered with reflecting layer 6 and Rayleigh scattering layer 7 so that no infra red light escapes through the bulb walls directly, the only loss in infra red energy being the loss due to the negligible scattering which occurs at Rayleigh scattering layer 7 and the small absorption of the infra red rays when they are reflected by reflecting layer 6.

It will be appreciated that although we have described specific embodiments of the invention, many modifications may be made, and we intend by the appended claims to cover all such modifications as fall within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. An incandescent lamp comprising a filament, a transparent evacuable envelope including at least one spherical segment having its center of curvature at said filament, an infrared reflector coating at least a portion of the interior surface of said spherical segment, and a Rayleigh scatterer comprising a thin particulate layer of a substance having irregular surface particles less than 3500 A. U. in dimension coating the interior surface of said reflector, at least a portion of said envelope being uncoated and remaining transparent to permit scattered light to escape from said lamp.

2. An incandescent lamp comprising a filament, a transparent evacuable envelope including at least one spherical segment having its center of curvature at said filament, reflecting means upon the 'mterior surface of said segment for returning thereto infrared radiation radiated from said filament and comprising a metallic reflecting film, and scattering means upon the interior surface of said reflecting means for causing Rayleigh scattering of visible light radiated by said filament and comprising a thin particulate layer-of a substance having irregular surfaced particles less than 3500 A. U. in dimension, at least a portion of said envelope being uncoated with said films and remaining transparent to permit scattered visible light to escape from said lamp.

3. The lamp of claim 2 wherein the light scattering means comprises a thin layer of particulate matter selected from the group consisting of magnesium oxide, zinc oxide, magnesium carbonate, finely divided silica, and finely divided titanium dioxide.

4. The lam of claim 2'wherein the light scattering means comprises a thin particulate layer of magnesium oxide.

5. An incandescent lamp comprising a transparent spherical envelope, a filament located at the center of said envelope, a metallic reflecting coating upon one surface of said spherical surface covering at least one half thereof, and a thin particulate Rayleigh scattering layer comprising irregular surfaced particles less than 3500 A. U. in dimension interior of and co-extendant with said reflecting coating.

6. The lamp of claim 5 wherein the Rayleigh scattering' layer comprises a particulate matter selected from the group consisting of magnesium oxide, zinc oxide, magnesium carbonate, finely divided silica, and finely divided titanium dioxide.

7. An incandescent lamp comprising a filament, an evacuable envelope including at least two spherical segments having centers of curvature located at said filament and at least one other segment joining the edges of said spherical segments, a light reflecting film coating the interior surface of said spherical surfaces but not the interior surface of said joining segment and a Rayleigh scattering layer comprising a thin particulate layer 'of a substance having irregular surface particles less than 3500 A. U. in dimension interior of and co-extendant with said reflecting film.

References Cited in the file of this patent UNITED STATES PATENTS 1,043,009 Hoffman Oct. 29, 1912 1,083,325- Hofiman Ian. 6, 1914 1,083,326 Hoffman Jan. 6, 1914 1,346,172 Bugbee et al. July 13, 1920 1,385,608 Darrah July 26, 1921 1,425,967 Hoflman Aug. 15, 1922 1,655,761 Duffy Jan. 10, 1928' 2,295,626 Beese Sept. 15, 1942 

